For the past few years, our research has been concentrated on a study of the role of the surface in the differentiation of the lymphoid system. In this respect, we have been concentrating on the immunogenetics of a number of cell surface markers specified by the Tla region of the mouse. This region determines a series of antigens called Qa which are predominantly represented on T cells but are present on B cells as well. In this pursuit, we have helped to identify and characterize five Qa alloantigens, all of which have unique characteristics. Qat-5, for example, is represented on a minor population of Ig minus peripheral lymphocytes and is absent from the thymus. Our more recent results indicate that there are still other Qa antigens, in addition to these five, which are determined by the Tla region.